1. Field of the Invention
The present invention relates to a method for calculating the heart rate variability of the human heart for implementation in an ECG monitor, and further to an ECG monitor having an operating program of which implements said calculation program. In this context, xe2x80x9cECG monitorxe2x80x9d is to be understood as any external or implantable device which detects and evaluates ECG signals, that is, apart from actual monitors, also for instance, the respective functional components included in pacemakers and defibrillators.
2. Background Art
Regarding the background of the invention, it has to be stated that heart rate variability is a risk indicator in the clinical practice with regard to patients having suffered from a cardiac infarction. It is therefore a declared objective of cardiology to record and evaluate the heart rate variability of such post-infarction patients in order to be able to initiate appropriate measures in case of dangerous values.
Another aspect in the context of the present invention is the trend to design cardiological monitoring devices for patients suffering from heart diseases in a way so as to make the device implantable into the body of the patient. When determining the heart rate variability, the frequency spectrum of the heart rate is determined, and the ratios of the maximum values are evaluated. Usually, the frequency spectrum is analysed using the so-called xe2x80x9cFourier transformationxe2x80x9d by scanning the ECG signal received by the ECG monitor during a specific scanning interval and determining from this signal a number of discrete measuring values representative of the heart rate variability, such as RR intervals.
As will be explained in greater detail in the description of the example embodiment with reference to the specific formulas, the frequency spectrum of the measuring values is calculated using the Fourier coefficients of the Fourier transformation. The Fourier coefficients themselves are calculated using a combination of the measuring values and the sinus- or cosinus-shaped Fourier vectors of the Fourier transformation. The problem of the usual Fourier transformation lies in the fact that for the mathematical implementation the (continuous) Fourier vectors have to be transformed into discrete support points. The support points are real figures reflecting the sinus- or cosinus-shaped course of the Fourier vectors. Depending upon the selected number of support points the number of multiplications required for calculating the Fourier coefficients from the Fourier vectors and the measuring values increases superproportionately. Therefore, the large number of multiplications using real figures requires an extreme calculating effort and an enormous need of memory capacity.
These high calculating and memory needs exceed by far the capacity of micro-processor systems available for implanted cardiological devices, thus rendering the application of a Fourier transformation for frequency analysis impossible as long as the prior art calculation methods and processor systems are used.
As a solution for these problems, the present invention suggests the replacement of the known (Fast) Fourier transformation in the analysis of the heart rate variability by an approximation method which, on the basis of the Fourier transformation, replaces the numerical vector values for calculating the frequency spectre of the scanned measuring values by a limited number of rough approximate vector values wherein these approximate vector values roughly reflect the course of the Fourier vectors. This method allows a considerable reduction of the calculating and memory needs and thus the application in implanted devices. The simplification is achieved by no longer presenting the Fourier vectors by support points of the sinus and cosinus functions in the form of real figures. Instead, said support points are selected as rough, integral approximate vector values, preferably based on the figures xe2x88x921, 0, and +1. A preferred upper limit for the number of approximate values may be five figure values xe2x88x922, xe2x88x921, 0, +1, +2 symmetrically surrounding the value 0. As a whole, this xe2x80x9crougheningxe2x80x9d of the support points generates a sort of xe2x80x9cfuzzy Fourier transformationxe2x80x9d.
Suitable evaluation results have been obtained when the limit for transforming the real numerical vector values into the approximate values xe2x88x921, 0, and +1 was +/xe2x88x920.33. A numerical vector value between xe2x88x920.33 and +0.33 results in an approximate value of 0 while numerical vector values  greater than +0.33 or  less than xe2x88x920.33 result in approximate values of +1 and xe2x88x921, respectively. As in the conventional Fourier transformation the sum of the approximate vector values for each Fourier vector has a value of 0, it is possible in case of a deviation of the sum of approximate vector values from the 0 value to correct at least one approximate value to obtain a sum value of 0. Thus, an ostensible offset in the analyzed frequency spectre and/or a wrong peak in the calculated frequency spectre at low frequencies are avoided.
The invention further relates to an EGG monitor comprising an operating program which implements the calculation method described above. Thus, said EGG monitor becomes implantable into the human body and/or can be integrated into a pacemaker.
Now, the calculation method according to the invention shall be explained in greater detail with respect to an example embodiment and with reference to the drawings attached hereto.